Automatic frequency control circuit



R. E. scHocK .AUTOMATIC FREQUNGY CONTROL CIRCYJIT Filed June 2e, 1957 Sept. 12, l 1939.

ATTORNEY Patented Sept. 12, 1939 UNITED STATES PATENT OFFICE Robert E. Schock, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application June 26, 1937, Serial No. 150,443

1 Claim.

My present invention relates to automatic frequency control circuits for radio receivers, and more particularly to signal-actuated regulation of the frequency control circuit.

In the past difculty has been experienced in the use of automatic frequency control (AFC) in radio receivers whenever the amplitude of the desired signal carrier decreased to, or near, the background noise level. In such cases the noise l energy, and sometimes adjacent channel signal energy, affected the AFC network in a sense to detune the receiver from the desired carrier frequency. Upon the desired signal carrier amplitude returning to a usable level, the receiver 15 was suiciently detuned to prevent operation of the AFC. In application Serial No. 16,591 of M. G. Crosby, filed April 16, 1935, Patent No. 2,- 123,716, July 12, 1938, there are disclosed, and claimed, several arrangements for automatically preventing AFC action when the strength of the incoming signal carrier decreases below a usable level.

One of the main objects of my present invention is to provide an AFC network for a radio receiver; and an additional circuit, responsive to signal carrier amplitude variation, being utilized to control the signal feed to the AFC network thereby providing a control over the possibility of noise energy, or adjacent channel signal energy, operating the AFC network when the desired signal carrier amplitude is below a desired level.

Another important object of my invention is to provide a receiver of the type employing signal-actuated means accurately to tune the receiver at different carrier frequencies; and additional means being employed to suppress the signal feed to the aforesaid accuracy tuning means when a desired carrier amplitude decreases below a predetermined level.

Another object of the invention is to provide in a superheterodyne receiver an AFC network; and means utilizing rectified signal energy to cut olf the signal supply to the AFC network whenever the signal to which the receiver is tuned drops below a predetermined level.

Still other objects of the invention are to improve generally the operating efficiency of receivers of the AFC type, and more especially to provide a signal responsive suppressor circuit for AFC receivers which is not only economical in construction, but reliable in operation.

The novel features which I believe to be characteristic of my invention 4are set forth in particularity in the appended claim; the invention (Cl. Z50- 20) itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organiza- 5 tion whereby my Vinvention may be carried into effect.

Referring now to the accompanying drawing, wherein there is shown in schematic manner a superheterodyne receiver, the signal collector A 10 may be of the grounded antenna type, a loop, a radio frequency distribution line, or the collector of a mobile vehicle such as an automobile. The collected signals, which may be in any desired frequency range, are impressed on a tunable 15 radio frequency amplifier I feeding a tunable rst detector network 2. The tunable local .oscillator network 3 feeds local oscillations to the rst detector; it will be understood that networks 2 and 3 may employ separate tubes, or they may use a 2O common pentagrid converter tube of the 2A'7 type. The numerals 4, 5 denote the usual variable tuning condensers whose rotors are arranged for unicontrol adjustment by a common means 6. Of course, the variabletuning condenser of 25 the first detector network will, also, be coupled to the tuning means 6 for adjustment.

The oscillator tank circuit 'l is tuned through a frequency range different from, and usually higher than, the signal circuit frequency range. 30 The signal energy in the output of network 2 is at an intermediate frequency (I. F.); the latter may be chosen from a range of to 450 kc. when the signal range is fro1n500 to 1500 kc. The I. F. energy is amplified by the amplifier 8; 35 the amplified I. F. energy in the output circuit 9 is impressed upon the networks to be described in detail. It will be understood that the I. F. transmission networks between the first detector 2 and the circuit 9 are each Xedly resonated to 40 the operating I. F.

The system shown is adaptable to unmodulated signal energy, or to signal energy modulated by any known type of modulation (amplitude; frequency; phase; or I. C. W.). Again, the receiver 45 may be of the type shown in said Crosby application. Regardless of the construction of the receiver prior to circuit 9, the I. F. Venergy is impressed on the input circuit l0 of a coupling tube Il. The circuit I0 is tuned to the operating 50 I. F.; and the signal grid I 2 of tube Il has the I. F. energy impressed thereon. Tube Il may be of the 6L7 type and has its cathode at ground potential; the plate I3 being connected Yto a source of positive potential (+B) through a path 55 including lead I4 and primary winding I5 of transformer M. The condenser I6 resonates the coil I5 to the operating I. F. The secondary winding I1 is resonated to the I. F. by the condenser I 8; the midpoint I9 of the winding I1 is connected to the high potential side of winding I5 by condenser 20.

The coupling tube has one of its inner grids 2| connected by lead 22 to the low alternating potential side of the input circuit I; the condenser 10 connecting lead 22 to ground. The signal grid I2 of tube II is connected by lead 23 to the high potential side of I. F. input circuit IIJ. The control grid I2 is electrostatically shielded from the grid 2| by a positive screen grid construction. I. F. energy is, also, impressed on the resonant vcircuit IU', which circuit is xedly tuned to the operating I. F.; the anode of a diode rectifier 24 beingv connected to the high potential side of circuit I0. The cathode of the diode is connected to the grounded side of circuit I0 through a load'resistor 25, the latter being shunted by an I. F. by-pass condenser 26.

The grid 2| of tube Il is Anot only connected by lead 22 to the circuit I0, but, also, to the cathode side of resistor 25; the negative biasing voltage source 21 being connected between lead 22 and the cathode side of resistor 25. It will be observed that the signal grid I2v of tube II is connected to the cathode side of resistor 25 through a path which includes lead 23, tuned circuit I0 and bias source 21. The function of the rectifier 24 is to produce a uni-directional Voltage across resistor 25 when the received carrier amplitude increases above a predetermined amplitude, and thereby overcome the initial cutvthe same winding.

off bias impressed on grids I2 and 2| by bias source 21. The bias source 21 can be supplied by any direct current potential source, such as properly filtered, rectified A. C., which will be independent of ground and be of low resistance. `When the I. F. 'amplitude is of suflicient strength to overcome the bias` from source 21, then the coupling tube II transmits I. F. energy to the discriminator network which is coupled to the circuit I6-I5. The discriminator network, and the frequency control tube 30, may be constructed as disclosed in applicationiSerial No. 45,413 of S. W. Seeley, filed Oct. 17, 1935, Patent No. 2,121,103, June 21, 1938.' Since the AFC arrangement now to be described has been fully disclosedin all its details in the Seeley application, it is not believed necessary to describe itin this application in more than general terms. Of course, it is to be understood that the AFC network may be of any well known form; for example, that disclosed in the aforesaid Crosby application may be utilized. All that is essential to a proper understanding of this application, is that it be understood that the I.A F. coupling tube II feed the discriminator of an lAFC arrangement, and that the coupling tube VIIA be dependent in transmission ec'iency upon the amplitude level of the I. F. carrier energy.

Referring now tothe discriminator network, the latter is of a type employing a pair of diodes 3| and 32; the anode of diode 3| being connected to one side c of winding I1, whereas the anode of diode 32 is connected to the other side d of The cathodes of the two diodes are connectedk in series by a pair of resistors 33 and 34, thevjunction point Dof the resistors being connectedto midpoint I9 of jwinding I1 through anI. F. choke coil; 35. The condenser 36 is connected across resistors 33 and 34, whereas condenser 31 is connected across resistor 34; the cathode side of resistor 34 is at ground potential. The cathode side of resistor 33 is connected by lead 40, the AFC lead, to a gain control electrode of the frequency control tube 30.

The AFC line 40 includes a lter network 4| for suppressing any pulsating components in the AFC' voltage. An AVC (automatic volume control) lead 59 may be connected to the point D for impressing AVC bias upon the I. F. amplifiers, and, if desired, upon any of the signal transmission stages preceding the amplifier 8. Furthermore, the audio Voltage component of the rectified I. F. energy may be taken off from point D, and transmitted through coupling condenser 60 to one, or more, stages of audio frequency amplication followed by a reproducer. It is to be understood, however, that the audio demodulator of the receiving system may be coupled to the I. F. circuit 9, and thus the second detection will be rendered independent of the discriminator action.

The frequency control tube 30 is connected to the tank circuit 1 of the local oscillator in such a manner that a variation of the gain of the control tube 3D results in a change in effective reactance (capacitative or inductive) in a sense such as to adjust the local oscillator frequency to maintain the desired I. F. value. The frequency control tube circuit may be of the type wherein the control tube has its input capacity shunted across the tank circuit 1; a change in gain of the control tube thereby varying the effective capacity reactance in the tank circuit. On the other hand, the control tube 30 may follow the teachings in the aforesaid Seeley application; and, if desired, the frequency control tube circuit used in the aforesaid Crosby application may be employed.

When the I. F. energy produced in circuit 9 varies in frequency from the operating I. F. value, then a bias will be developed by the discriminator network; the bias is transmitted through line 40 to the frequency control tube 30, and the latter is varied in gain so as to adjust the tuning of tank circuit 1 in a sense to maintain the operating I. F. In this way whenever the tuning device 6 is` adjusted. to a tuning position corresponding to a desired station, the reception of signal carrier energy above a predetermined amplitude results in actuation of the discriminator network so as to cause thereceiver to be pulled into tune with the desired station.

The I. F. carrier is delivered by coupling tube II to circuit I5-I6. The coil I5 is coupled loosely to circuit I1-I8. The point a of circuit Iii-I3 is connected to the midpoint I9 of the winding I1, it being understood that condenser 29 is assume-d to be so large that the voltage drop in it is negligible. The voltage at the cathode side of resistor` 33V will be either positive or negative with respect to the grounded sideof resistor 34, depending Von which way the selector circuits are detuned. The point 0Lv and point I9 are at the same alternating potential because of the large magnitude of condenser 20. The phase of point a. with respect to ground potential is Zero, when the I. F. energy is at the operating I. F. At resonance there is no phase shift in the circuit |5-I6. Hence, point|9 is at zero phase. The current in circuit |5-I6 induces a voltage in circuit I1I8, and this voltage is distributed equally about point I9. At

a given instant point c is as much positive as point d is negative. The voltages impressed on the two rectiers are therefore equal, although opposite in phase. The rectified outputs depend only on the magnitudes, and, hence, the Voltage drops across 33 and 34 will be equal. Since rectiers 3| and 32 are connected in series opposing relation, the potential difference between ground and the cathode side of resistor 33 will be zero. This balance occurs only when the frequency is equal to the resonant frequency of the two loosely coupled I. F. circuits.

If, now, the signal frequency differs considerably from the resonant frequency, there will be a phase shift of nearly 90 degrees in the circuit. The voltages induced in the two halves of the secondary I1 are still equal in magnitude, and opposite in phase with respect to point I9. The voltage drop across circuit I5-I6 is now added vectorially to the voltages induced in circuit I1I8. Thus, the potential at one side of the secondary I1, say c, will be the sum of the induced voltage (I S-o) and the voltage drop across I5-I6; while the potential of the other side, d, will be the difference between the drop in I5-I6 and the Voltage induced in (I 9-d). It follows that the input voltage of one rectifier, the upper one in the assumed case, is much greater than that in the other. Therefore, the voltage drop across resistor 33 will be `greater than that across resistor 34, and the cathode side of resistor 33 will be positive with respect to ground.

Depending on the sense of frequency departure of the I. F. energy from the operating I. F., the cathode side of resistor 33 `assumes either a positive or negative potential with respect to ground. The magnitude of this AFC bias depends on the amount of frequency departure. 'Ihe AFC bias determines, in this way, the gain of the control tube 30; as Well as the sense of gain change. This, in turn, determines the magnitude and sense of change of the simulated reactance across the tank circuit 1. Point D is always negative relative to ground; hence, the AVC line 5ll is connected to this point. Of course, AVC bias and audio voltage can be derived from a separate detector coupled to I. F. circuit 9.

The bias source 21 is preferably adjusted to impress a negative bias on grids I2 and 2| which is of sufficient magnitude to bias coupling tube I I to cut-off when the I. F. energy impressed on circuit I9' is of an amplitude below a usable value. As the I. F. carrier energy increases in amplitude to the value above the desired level, the direct current voltage dro-p across resistor increases in a positive potential sense to a point such that it overcomes the negative bias from source 21 to an extent sufficient to permit normal operation of the coupling tube II. Of course, sufficient bias will then be supplied by source 21 to permit the tube I I to operate as an eflicient I. F. amplifier. It

will, therefore, be seen that the magnitude of the voltage from source 21 determines the level of signal required before the tube I I will pass a signal, and that this level of operation may be predetermined.

It is to be understood that the circuits I0 and IG need not be fed from the same source, or by the same frequency. For example, the frequency discriminator may be one which operates at audio frequency, and circuit I0 may be an audio frequency circuit fed by audio frequency energy which has been heterodyned from the radio or .intermediate frequency signal in the receiver under frequency control. At the same time, the circuit IS may utilize a radio, or intermediate, frequency signal. The circuit I 0 may be fed with any of the I. F. energies in a multiple detection heterodyne receiver; or it may be fed with heterodyned audio frequency energy. Again, while the coupling tube I I has been shown of the multi-grid type employing a control voltage on two grids, it is to be clearly understood that the control bias may be applied to one control grid, and that, in general, any type of tube can be employed in place of tube II as long as its signal transmission eiliciency is greatly minimized when the signal amplitude falls too low.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made Without departing from the scope of my invention, as set forth in the appended claim.

What I claim is:

In a superheterodyne receiver of the type including a tunable local oscillator, an intermediate frequency transmission network and an automatic frequency control circuit, responsive to a frequency departure of the intermediate frequency energy from an .assigned frequency value, for controlling the oscillator frequency; the improvement comprising a tube provided with a cathode, anode and at least two cold electrodes, means impressing intermediate frequency energy upon one cold electrode, means coupling the anode to said control circuit for feeding said intermediate energy thereto, means responsive to amplitude variation of said intermediate energy for automatically controlling the bias of the second cold electrode thereby to control the gain of said tube, said last means consisting of a diode rectier having an input circuit coupled to said transmission network, said rectier including a load resistor, and a connection, including a source of voltage normally adapted to cut-off said tube, between said second cold electrode and a point on said resistor adapted to assume a positive potential.

ROBERT E. SCHOCK. 

